In 100 Megabit per second (Mbps) Fast Ethernet applications, a 100 Mbps transmitter transmits a multi-level transmission-3 (MLT-3) signal through a coupling transformer to a transmission medium, and then a 100 Mbps receiver receives the MLT-3 signal over the transmission medium. The transformer is equivalent to a high-pass filter which blocks a DC component of the MLT-3 signal. Unlike the Manchester data encoding scheme used in 10 Mbps Ethernet systems, the MLT-3 signal is not DC balanced and its DC component varies with the signal pattern. When the DC component is filtered, it cannot be compensated sufficiently by simply adding a common mode voltage with a fixed DC level in the receiver end. Thus, an undesirable phenomenon known as baseline wander occurs. If baseline wander is not cancelled out or compensated for, the phenomenon can cause signal distortion in the front end of the receiver. In the worst case, baseline wander can cause the back end of the receiver to produce incorrect results.
FIG. 1 shows a data transmitter baseline wander correction circuit according to the prior art. It uses a feedback circuit taking a feedback signal Efeedback from one of the windings of a coupling transformer to generate an estimated DC value. Then, the estimated DC value is added to a digital signal to be transmitted. Thereby, the baseline wander problem in the output transmit signal is corrected. However, a drawback of the data transmitter of FIG. 1 is that it is possible for the feedback network to become unstable. Moreover, if the coupling transformer is not matched well, the output transmit signal and the feedback signal will not be the same. Even though there is no baseline wander in the output transmit signal, it cannot ensure that the receive transformer coupled to the transmission medium does not introduce the undesirable phenomenon at receiver end.
Another technique for correcting baseline wander is illustrated in FIG. 2. By utilizing a peak detector to detect possible directions of baseline wander, the DC value of the received signal is thus adjusted to compensate for baseline wander. A disadvantage of the technique of FIG. 2 is that the output of the peak detector can be affected by data patterns. This may cause ripples in the detected output so that the system of FIG.2 cannot achieve perfect baseline wander correction. Also, the received signal passes through an equalizer prior to correcting baseline wander. Hence, the linearity required for input terminals of the equalizer should be very strict.
Accordingly, what is needed is a novel technique for correcting baseline wander that utilizes characteristics of baseline wander to compensate for the undesirable phenomenon of a received signal before the signal passes through an equalizer. It would be highly preferable for such a technique to be immune to the effects of the received signal peaks. Further, it would be desirable to have a receiver with baseline wander correction that decreases its production costs.